A typical application of this type of circuit arrangement and method is in the control of injection valves for high-pressure diesel injection systems. These systems are subject to certain manufacturing tolerances which affect the opening and closing times of the valves as well as the throughput of fuel. Depending on the very high fuel pressures (up to 1600 bar), the in some cases very short injection durations and the extremely minimal injection amounts, even small manufacturing tolerances cause problems with the exact dosing of the injection amount and thus lead to an adverse effect on performance and motor noise. It can also lead to less complete combustion and thus to increased smoke generation. More precise manufacturing is in most cases not possible or is only possible with unacceptably high increases in costs. Conversely however it is possible, by appropriately adapting the individual control parameters, to compensate for manufacturing tolerances during operation.
Manufacturers thus resort to calibrating the injection valves individually during manufacturing and classifying them in accordance with their behavior. To identify the classification each valve is assigned a calibration resistor in the unit of which the resistance represents the corresponding classification. Determining the resistance value of the calibration resistor and comparing it with a corresponding classification list allows suitable control parameters to be found with which the individual deviations from the norm of the valve concerned can be compensated for in such a way that a desired operating behavior, e.g. as regards injection time, duration and/or quantity, is achieved.
This can be implemented in modern, microcontroller-based systems, on initialization of the valves, before the motor is started, by individually determining the valve calibration resistances, calculating the assigned control parameters or reading them out from a memory and by the control software taking them into account during operation. This avoids the measurement being affected by normal valve operation and also records the fact that a valve may have been replaced, e.g. within the context of a repair.
To avoid additional cabling the calibration resistor is often installed in conjunction with an activation coil of the valve or, with bipolar control, between the activation coils, in which case switching means, in particular transistor circuits, are provided which switch backwards and forwards between and initialization configuration and an operating configuration of the overall circuit. The calibration resistance can thus be recorded when the coil control is switched off by the evaluation circuit present in the initialization configuration.
FIG. 3 shows a basic diagram of this type of evaluation circuit according to the prior art, restricted to an individual valve, in which, to aid clarity, the switching means as well as the control of the latter are not shown. Controlled by a decoder (not shown), a series resistance circuit made up of three resistors is connected via two transistors (not shown) and four resistors (not shown) to the supply voltage of the 48 V on-board network. The connection of the first two resistors is connected to an opener coil (not shown) of a bipolar valve and thereby to the calibration resistor assigned to the valve. With further switching means (not shown) which include the above-mentioned decoder, an OR gate and also a transistor, the calibration resistor is connected to ground at the other end. The circuit is thus essentially a voltage divider circuit with the calibration resistor as an additional load resistance. In this case the voltage is tapped off between the second and the third resistance of the series resistance circuit, to divide the tapped voltage down into a value which is suitable as an input voltage for an analog multiplexer. This type of component is provided in order to accept the calibration voltages for all valves of the system in turn and switch them through to its output. The latter is connected to an appropriate analog/digital converter (ADC) input of a microcontroller (not shown). As soon as a calibration voltage is read in by the ADC of the microcontroller, the switching means switches over to the next valve or the next calibration resistor and the described procedure is repeated until the classifications of all valves are read in.
The disadvantage in this arrangement and in the corresponding procedure is the extremely high switching overhead produced by the multiplicity of components needed. For an 8-cylinder engine for example eight valves each with an evaluation circuit as described above are required. As well as the high costs, another underlying problem here is a corresponding reliability risk and the system causes space problems which can only be compensated for by an appropriately more complex and thereby expensive board layout.